Ultra high frequency rotatable joint



`une 14, 1949. G. RAGAN ULTRA HIGH FREQUENCY ROTATABLE JOINT Filed April 18, 1944 n su INVENTOR GEORGE L RA GAN BY JWM Patented June 14, 1949 2,473,443 ULTRA HIGH FREQUENCY ROTATABLE JOINT George L. Bagan, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Appication April 18, 1944, Serial No. 531,607

8 Claims.

This invention relates to a joint for high frequency conductors, and particularly to a rotatable joint comprising a coupling between a rectangular wave guide and a coaxial line disposed at right angles to the guide.

The principal object of the invention is to provide a right angle rotatable joint of this character wherein the conductor for transmitting wave energy will be changed from rectangular to coaxial without substantial losses due to reflections. Another object contemplates a rotatable joint wherein a substantially good low resistance path for high frequency currents is provided without the use of relatively moving contacts. The joint also is substantially free from losses due to radiations. A further object is to provide a joint where one or more concentric coaxial cables may be rotatably associated with a stationary wave guide, the respective coaxial cables being adapted to rotate at different speeds. This feature is useful in instances where more than one line is required for a rotating device such as an antenna.

In the drawing, Fig. 1 is a sectional view on a diametral plane through a rotatable joint, and Fig. 2 is a sectional view on the line 2 2 of Fig. 1 which is partly broken away for clarity.

Referring to Fig. 1, a rectangular wave guide 5 has opposite b (Wide) sides 6 and 'i and a (narrow) sides 8, constructed in the usual manner and with predetermined ratio between b and a sides. A circular opening is provided in wide side 6, and a coaxial cable housing Iil, extending at right angles to said wide side 6 of the guide, is rigidly secured to said side in aligned relation with said opening. The diameter of the circular opening may be of the order of half the width of side 6, this dimension being generally determined by a launching of the electric field in a manner hereinafter described. Housing I@ is preferably cylindrical in shape, the end thereof afxed to the guide being reduced in section to provide interior annular shoulder II.

A coaxial cable, indicated generally by I5, having a tubular central conductor Iii and spaced outer conductor I 1, is mounted for rotation within housing IU. Central conductor l5 is maintained within outer conductor Il in any7 usual manner, as for example by means of insulating spacer beads I8, or, if desired, conductor I5 may be free to rotate with respect to conductor l'l. A metallic sleeve I9 snugly encircles outer conductor I'l for a, substantiallength within the housing Iil and is secured thereto in any suitable manner as by solder. A cylindrical bearing spacer 2i) is rigidly associated with sleeve I9, means such as set screws 2| being provided to fix the two together. A bearing sleeve 22 lies against the interior oi the housing Il), in opposed relation to bearing spacer 20, and a plurality of roller bearings 23 and 24 carried in annular retainers 25 and 28 are disposed between bearing spacer 2l) and opposing sleeve 22. Ring 2l serves to space bearing supports 25 and 26 properly with respect to each other. Thus coaxial cable I5 is rotatably associated with housing II).

Referring to the lower end of housing I, an annular gasket 3D having ring seal 3l is disposed between opposing gasket retainers 32 and 33, the latter lying against housing shoulder II. Retainer 32 bears against the lower ends of spacer 2D, sleeve 22 and annular bearing retainer 25. Gasket 30, which may be of the usual neoprene (rubber) type, extends between the sleeve I9 and housing Il) to seal the rotatable electric joint from the mechanical bearing assembly, thus to prevent dirt and oil from entering to cause possible arcing.

In the upper end of housing I, a gasket retainer 46 spaces the bearing assembly from annular gaskets 4I and 42. These gaskets, which have ring seals 43' and 44, respectively, extends between conductor sleeve IS and housing Il! to seal the joint in a pressure-proof manner. Gaskets 4I and 42 likewise may be made of neoprene. Upper gasket retainer 46 loosely encircles sleeve I9 and provides a bearing surface for inwardly extending flange 47 of assembly nut 48. Nut 48 may be detachably associated with the end of housing I0 by a threaded connection, as shown in the drawing.

Central conductor IB of coaxial cable I5 extends through the space within guide 5. Conductor I6, if desired, may also constitute the outer conductor lof a concentric coaxial cable having an inner conductor 43. This latter conductor may be suitably mounted for rotation relative to conductor I6. If such a secondary coaxial line is provided, conductor I5 may project beyond guide side l, a circular opening being provided in the side to pass the conductor, as shown in the drawing.

A metallic plate 5l) is rigidly secured to the inner surface of guide side 7 around the abovementioned opening, a depending annular fia-nge 5I thereof being snugly received in said opening. Flange 5I defines a central -opening within which is mounted an annular bearing 52 journaled about the opposed end of conductor I6. An annular ange 53 may be provided on conductor I6 to cooperate with bearing 52 in order to position the conductor axially within the joint. Thus con- 3 ductor I6 is adapted to rotate freely in bearing 52. An annular fitting 54 is disposed about and spaced from conductor I6 within wave guide 5, the fitting being tapered from a narrow diameter at the top adjacent the circular opening in side 6 to a diameter at the bottom corresponding substantially to the diameter of plate 50 with which lt is associated. Fitting 54 cooperates with the electrical joint in a manner presently to be described. A cap 55 surrounds conductor I6 and encloses the opening in side l. Cap 55 is secured to the side 'I by bolts 56 and 51 extending through the cap, guide side l, plate 5U and threaded into fitting 54. Rubber gaskets 58 and 58a may be used in conjunction with cap 55, if the system is to be kept under pressure.

Wave guide 5 has its end beyond the joint structure closed by a seal 59 and rounded in accordance with the shape of associated plate 50 and fitting 54, best shown in Fig. 2.

Metallic fitting 54,'disposed in the wave guide 5 in aligned relation with coaxial cable I5, tends to concentrate the electric field. For this reason the coaxial cable is somewhat smaller than the width of associated guide side 6.

In order to provide a low impedance at the junction of the two conductors, fitting 54 has an interior annular channel 60 constituting part of a resonant cavity. The depth of channel 66 from its end 6I adjacent bearing 52 to its inner end 62 is generally a quarter wave length of the transmitted frequency. Channel 6! communicates with annular. free space 63 defined by conductor I6 and spaced fitting 54, the depth of space 63 from common end 6I to its opposite end S4 being likewise a quarter wave length. Thus channel 60 and space 63 together comprise a half wave resonant cavity terminating at end 64. Such a cavity provides a path of low impedance to high frequency currents between fitting 54 and rotatable central conductor I6, thus permitting a smooth flow of energy between the wave guide and the coaxial line. In this manner, substantial losses due to refiections are avoided. In addition, journalled bearing 52 is at the quarter wave point of the cavity, and thus at a substantially open circuit point in the system. Therefore, little or no current theoretically passes between the rotating conductor surfaces. In practice and because of variations in frequency, some currents may pass, and for this reason it may be desirable to run the bearing dry or with graphite rather than oil which may be carbonized by current arcs.

An annular choke structure 65 is disposed within the reduced end of housing I0. Structure 65, which comprises a stationary section of outer conductor in the coaxial cable, has an upstanding annular flange 66 spaced from the housing wall to define an annular quarter wave cavity 61. This cavity acts as a choke to prevent radiation and consequent loss in a direction normal to the coaxial cable. Flange 66 is spaced from outer conductor I'I of the rotatable coaxial cable to define a communicating cavity 68, also a quarter wave in length. Cavities 6'I and 68 thus form a half wave resonant cavity which reflects a low impedance for high frequencies to thev discontinuity point between conductor I1 and structure 65. Consequently there is a, substantially refiection-free flow of energy between these spaced conductors. It will be noted that the interior surfaces of conductor I'I and structure 65 preferably lie in the same cylindrical plane at the spaced junction. The interior annular surface of struc.. ture 65 away from the junction is rounded, as

shown at 69, this surface corresponding generally in shape to opposing surface 'IIJ of tapered fitting 54. These smooth opposing surfaces shape the electric eld for proper orientation with respect to the two coupled conductors.

Thus the present invention provides a rotatable joint constituting a high frequency transition section between a rectangular wave guide and a coaxial line. The two'condictors are coupled at right angles to each other, and regions of low impedance are provided to minimize reflections d ue to the generally sharp impedance discontinuity present in such a coupling. Low impedances are obtained by the use of half wave resonant cavities disposed in the joint in such a manner as to reflect low impedance to the spaced junctions of the conductors. In this manner a substantially continuous non-metallic circuit is provided in the joint. Also, the relatively rotating contact surfaces lying in the path of current flow are positioned at the quarter wave point with reference to one of the cavities, whereby current flow between such surfaces is a minimum. Further, a quarter wave choke is provided to prevent radiations and consequent losses radially o f the coaxial cable. The joint also is adapted to contain more than one coaxial line if desired. Such additional lines may be concentrically mounted for rotation within the primary coaxial cable. Inasmuch as several lines may be necessary to a device such as a rotating antenna, the range of usefulness of the present invention is substantially increased.

Having thus described the invention, what I claim as new and desire to secure by Letters Patent is:

1. In a high frequency transmission system, a rotatable joint construction comprising a rectangular wave guide having a sealed end to close the same, said wave guide having a, circular aperture in one wide side adjacent said sealed end, a circular conductor supported at said aperture and eX- tending at right angles from said wave guide and forming one section of outer conductor for a co-V axial line, a second section of circular conductor aligned with the first section, means for rotatably supporting said second section with respect to said first section, a central 'conductor for said coaxial line, said central conductor having one end extending through said wave guide aperture at least to the opposite side of said wave guide, means for rotatably supporting said end of said central conductor, and a fitting carried by said opposite side of said wave guide for providing a transition structure between said opposite side of said wave guide andthe said central conductor, said fitting comprising a stationary annular structure surrounding said central conductor and spaced therefrom, said fitting extending from said opposite side of the wave guide to a termination near the aperture in said first side of said wave Vguide and tapering to a smaller diameter as said aperture is approached, said fitting having an internal annularchannel communicating with the free space between said fitting and said central conductor, said annular channel and said free space togetherl having an effective length of substantially a, half wave length whereby a good low' impedance rpath is provided for high frequency energy between said stationary fitting and saidcentral rotatable conductor.

2. The combination of claim 1 wherein a tubular housing encircles and is spaced from said second section, andy said lrst section of outer conductor comprises lan annular structure disfposed about said circular aperture, said struce ture having an annular flange extending between and spaced from said second section of coaxial cable and said housing and dening a half wave resonant cavity, one quarter wave of which is on each side of said flange, thus to provide a choke to radiation in a direction normal to said line.

3. The combination of claim 1 wherein a tubular housing encircles and is spaced from said second section, and said first section of outer conductor comprises an annular structure associated with the guide side about said circular aperture, said structure having an annular flange disposed between and spaced from Said housing and said second section of circular conductor, said flange, housing and second section dening therebetween a half wave resonant cavity whereby a low impedance path for high frequency energy is provided between said stationary first section and said rotatable second section.

4. The combination of claim l wherein said means for rotatably supporting the end of said central conductor is disposed proximate to the quarter wave point of the said annular channel and communicating free space between said ntting and said central conductor, whereby a substantially high impedance is provided at said means.

5. The combination of claim 1 wherein said central conductor extends beyond said wave guide through an aligned opening in said opposite guide side, said central conductor being tubular, and there are also a second central conductor coaxial with and of lesser diameter than said first central conductor, and means for rotatably supporting said second central conductor with respect to at least one of the other conductors.

6. A rotatable joint, comprising a stationary stantially coaxial with said opening, the axis of V said first section oi outer conductor being sub stantially normal to the axis of said wave guide, a smooth annular iirst transition surface intermediate said rst wall and said rst section oi outer conductor, a second section of outer tubular conductor for a coaxial line having an inside dimension similar to said rst section and disposed coaxially with said rst section, the adjacent ends of said rst and second sections of outer conductor being spaced from each other so as to form a rst gap therebetween, means to sustain said second section of outer conductor to allow it to rotate about its axis, an inner conductor for a coaxial line extending through said first and second sections of outer conductor and coaxial therewith, means to sustain said inner conductor to allow it to rotate about its axis, a tting secured to said second Wall coaxial with said inner conductor and extending toward the opening in said first wall, the outer surface of said fitting forming a substantially smooth second transition surface intermediate the second wall of said wave guide and said inner conductor, the edge of said second transition surface adjacent said inner conductor being spaced therefrom so as to form a second gap therebetween, first half wave resonant cavity terminating at said first gap and presenting a low impedance at said iirst gap, and a second half wave resonant cavity terminating at said second gap and presenting a low impedance at said second gap.

7. A rotatable joint as defined in claim 6, wherein said rst resonant cavity has a quarter wave point adjacent the means to sustain the second section of outer conductor so as to present a high impedance to curtail electrical losses, and wherein said second resonant cavity has a quarter wave point adjacent the means to sustain the inner conductor so as to present a high impedance to curtail electrical losses.

8. In a high frequency transmission system, a rotatable joint construction, comprising a wave guide, said wave guide having an aperture in one side, a tubular conductor supported at said aper ture and extending substantially at right angles to said wave guide and forming one section of outer tubular conductor for a coaxial line, a second section of tubular conductor aligned with said first section, means for rotatably supporting said second section with respect to said rst section, a central conductor for said coaxial line, said central conductor having one end extending through said wave guide aperture and toward the opposite side of said wave guide, means for rotatably supporting said central conductor, and a iitting carried by said opposite side of said wave guide for providing a transition structure between said opposite side of said wave guide and said central conductor, said fitting comprising a stationary annular structure surrounding said central conductor and spaced therefrom, said fitting extending from said opposite side of the wave guide to a termination near the aperture in said rst side of said wave guide and tapering to a smaller diameter as said aperture is approached, said fitting having an internal annular channel communicating with the free space between said fitting and said central conductor, said annular channel and said free space together having an effective length of substantially a hallV- wave length whereby a good low impedance path is provided for high frequency energy between said stationary iitting and said central rotatable conductor.

GEORGE L. RAGAN.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,030,179 Potter Feb. 11, 1936 2,223,224 Newhouse Nov. 26, 1940 2,434,925 Haxby Jan. 28, 1948 

